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Thread: synchronizing Bing carbs

  1. #1
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    synchronizing Bing carbs

    Hello , I have just rebuilt the Bing 94 Carbs on my 1980 R100RT , I changed the cables also and the bike starts and runs but has a noticeable vibration.
    so now I want to adjust the synch. of the carburetors. Reading the various manuals and as I understand it my basic set up is with the idle stops screwed in one turn from just touching the lever and the idle mix screw one turn out from a gentle seating. Then the cables , when attached should have 1/2 mm of loose play at the carbs. this being both the throttle and the enrichener. I have two single throttle cables no 'Tee'.
    and now the question is which method of synchronizing shall I use?
    I have read snobum and feel that I should buy a vacuum synchronizer but which one? there are a couple in the Bing catalogue 'Carb mate' and 'Synchromate II' I am sure there are many more any one have any advice here for the amateur, much appreciated. Thank you Heed

  2. #2
    Administrator 20774's Avatar
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    The enrichener cables don't have any play...they need to be tight and completely off.

    Nothing wrong with the Snowbum method. You just have to make positive connections between the engine fins and the grounding spoke to effectively shut off the cylinder. I also have the TwinMax...I've used it and like it.

    One thing that you can do without the shorting method which will cut off the cylinder. First do the idle mixture which doesn't require shorting or any instruments. Then for the idle speed synch, turn the mixture screw all the way in until it seats, noting exactly the number of turns you made. That cuts off the gas and stops the cylinder. Make any adjustments...then turn the mixture screw back out exactly the same number of turns. Repeat on the other side. The same can be done for setting the throttle cable tension with the engine running around 1500-2000 RPM...shut off the mixture screws...I think that will be enough to cause the cylinder to stop running...if the RPM is too much higher, then it won't be drawing from the idle circuit as much.

    The throttle cable tension can also be checked by putting your head between the two exhaust pipes and have someone slowly open the throttle. You should be able to hear which side picks up first. That's the side that has too much tension in the cable...loosen it. Should get you pretty close.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    Registered User jad01's Avatar
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    I debated about buying a tool like the TwinMax, Carb Mate, etc., but was put off by the cost. I ended up building a manometer that does the same task... much less expensive and works very well. There was a recent thread with a few examples, links, etc: http://www.bmwmoa.org/forum/showthre...meter-question.

    Good luck!
    Jim
    '78 R80/7 and '84 R100RS (Blues Brothers), '86 K75C (Icy Hot)
    '90 and '93 Mazda Miatas (Jelly Bean and Red Hot), '02 325ci (Blue Streak)
    '96 Giant Upland (big Kendas & freshly greased bearings!)

  4. #4
    Administrator 20774's Avatar
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    I disagree with a couple of things in the procedure that brittrunyon posted. It has to do with the tests for throttle tension starting with #13. Generally, it's not good to do the tests at such a high RPM...it's better to do it at a lower RPM where the butterfly is only slightly open and where small changes will have a bigger effect. This gives you good balance coming off idle...at the higher RPMs, there is so much going on, you won't notice the changes as well. It's basically a compromise anyway...it'll never ben in balance throughout the entire range.

    The other thing that I question is where it says to take up slack in the weak cylinder. In fact, you want to induce slack in the strong cylinder. Think about your initial settings. You put a small amount of slack in the cables so you could set the idle speed. If now later in the cable test, you reduce that slack, you might reduce it so much as to mess up your idle settings. It's best to add slack to the strong side.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  5. #5
    '92 R100GS '81 R100/t brittrunyon's Avatar
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    Quote Originally Posted by 20774 View Post
    I disagree with a couple of things in the procedure that brittrunyon posted. It has to do with the tests for throttle tension starting with #13. Generally, it's not good to do the tests at such a high RPM...it's better to do it at a lower RPM where the butterfly is only slightly open and where small changes will have a bigger effect. This gives you good balance coming off idle...at the higher RPMs, there is so much going on, you won't notice the changes as well. It's basically a compromise anyway...it'll never ben in balance throughout the entire range.

    The other thing that I question is where it says to take up slack in the weak cylinder. In fact, you want to induce slack in the strong cylinder. Think about your initial settings. You put a small amount of slack in the cables so you could set the idle speed. If now later in the cable test, you reduce that slack, you might reduce it so much as to mess up your idle settings. It's best to add slack to the strong side.
    Kurt,
    2 excellent points.

    In regard to #13 what rpm do you think, 1,700?

    In regard to reducing or adding slack, I think that could work either way, depending on the amount of original slack.

    It may be hard to believe but I enjoy balancing the carbs. It's very satisfying once you get them there. You hear it, you feel it, you ride it
    1992 R100 GS, 1981 R100/t, 2007 F 650 GS

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  6. #6
    Administrator 20774's Avatar
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    Quote Originally Posted by brittrunyon View Post
    In regard to #13 what rpm do you think, 1,700?

    In regard to reducing or adding slack, I think that could work either way, depending on the amount of original slack.
    I think I've seen 1500-2000 recommended. In terms of reducing slack, sure you could do that, but then you'll be required to go back and confirm that you do indeed have slack at idle. And if you've reduced the slack, what will it be after a 300 mile day? I think the initial slack is there to ensure that under all conditions, you will still have slack. It's hard to know that when doing a carb synch after a 15-20 minute ride!
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  7. #7
    Benchwrenching PGlaves's Avatar
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    Quote Originally Posted by brittrunyon View Post

    In regard to #13 what rpm do you think, 1,700?
    As close to idle speed as practical, provided you are off the idle stops and on the cables. This provides the largest change in inlet cross section (and thus change in vacuum) for a given change in throttle plate angle.

    I'd say about 200 rpm above idle speed is easy to maintain, and ensure adjusting the cable doesn't drop you back to the idle stop screw.
    Paul Glaves - "Big Bend", Texas U.S.A
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  8. #8
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    I've used both Carbtune & Mercury Sticks. Both work well for sync work and the Carbtune with no Mercury is safer.

    With respect to the higher RPM adjustment. On my oilhead the 3000 rpm was important as you could get a good sync at idle but then when you raised the revs it would change. I always set the sync for the best reading at 3000 rpm and didn't worry too much about what they were at idle, as the bike was only there at startup. I use the same practice with my airhead. Worked fine with both EFI & carbs. It only takes a couple of seconds to check the sync at 3000 and I always consider it the final adjustment in the sync process.

    Using the higher rpm point will make the engine run smoother in the cruising range.
    1971 R50/5 SWB with R75/6 drivetrain
    2013 DL650

  9. #9
    #4869 DennisDarrow's Avatar
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    Just a couple of quick things...........

    For me, I ALWAYS warm up the bike by taking it on a 20 mile or so ride. Have tried just warming it so the chokes dont do their job anymore and adjust then and find that when I really get it warmed up later that the idle is totally offf..............

    To help roll on stumble I totally agree about adding just a bit of tightening to the weak side throttle cable.

    Having used the mercury, electronic, by ear, killing one side by grounding the plug, and the el cheapo method of tubing and ATF for the fluid it comes down to what works for you and what you have available. Using the vacuum to get the feel and learn how and what it sounds like is valid and then when you have to use the shorting or by ear methods.....................

    For me.........yes, idle sync or just beginning to roll on is very important; but I RIDE at 3000 RPM and up. You will see that these readings on a vacuum metering system vary greatly at idle++ and 3000........SO, yes, I get the idle++ all synced up and then crank it up to 3K and adjust the one side or other throttle cable that needs it...........

    USE THE BIGGEST FLOOR FAN YOU HAVE while doing all of this. Set it in front of the bike blowing into the engine and across the cylinders...........


    Good luck and God bless........Dennis

  10. #10
    Registered User 88bmwjeff's Avatar
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    I don't understand #5. Granted I'm no expert or guru. I thought the purpose of the X number of turns out is to set the carbs at reasonable basic settings to get the bike started, so you can properly adjust the settings. If already on the bike, the place where they are at now would be the best starting place to adjust from. Just my opinion. Same with #6, IMO.

    Edit: I don't mean to be slamming the suggestions. In fact, the write up provides plenty of good suggestions and procedures. I just think steps 5 and 6 are not necessary.
    Jeff in W.C.
    1988 R100 RT (the other woman)
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  11. #11
    Administrator 20774's Avatar
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    Jeff -

    While I typically don't do #5 or 6, I can see the point in that you put each carb at some known point and go from there. What has happened to the settings over the 2-3 years since the last time you adjusted the carbs? If you happen to be adjust mixture wise too far, you might continue to adjust beyond that and when things don't respond correctly, you stand back and say WTF. So, starting from the stock position could make sense, generally. Plus, the stock positions should be just about right most of the time. If during the adjustments, you have to go two extra turns out or something, you might want to stop and think about that to figure out why it's so far away from the stock position.

    Note that those instructions were written as quasi dummy proof. All of the suggestions that we've put in this post have likely left out key steps because we've done it so many times we just assume everyone else knows that!
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  12. #12
    '92 R100GS '81 R100/t brittrunyon's Avatar
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    Quote Originally Posted by 20774 View Post
    Jeff -

    While I typically don't do #5 or 6, I can see the point in that you put each carb at some known point and go from there. What has happened to the settings over the 2-3 years since the last time you adjusted the carbs? If you happen to be adjust mixture wise too far, you might continue to adjust beyond that and when things don't respond correctly, you stand back and say WTF. So, starting from the stock position could make sense, generally. Plus, the stock positions should be just about right most of the time. If during the adjustments, you have to go two extra turns out or something, you might want to stop and think about that to figure out why it's so far away from the stock position.

    Note that those instructions were written as quasi dummy proof. All of the suggestions that we've put in this post have likely left out key steps because we've done it so many times we just assume everyone else knows that!
    Well said................

    Oh yea, I'm the quasi dummy...............
    Last edited by brittrunyon; 05-31-2013 at 01:40 AM. Reason: Just remembered........
    1992 R100 GS, 1981 R100/t, 2007 F 650 GS

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  13. #13
    R100GS, '89 Guenther's Avatar
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    I use a statice method:

    I disconnect each carb from the cylinder and the intake pipe. I point a flashlight from ome sight and check for a sliver of light on the other side when increasing the idle stop screw from fully close. Then I turn the screw back so I just see that the throttle plate just barely closed. I then open the idle scew by 1 full turn. Now I have my synched idle position for both carbs.

    Next I attach the carbs and set the idle enrichment screws to 3/4 open and start the engine. If the idle is too low I increase the idle by turning up the idlle screws on both sides by an exact half turn.

    Once it runs on idle somehow I adjust the idle enricher screws up/down on each side until I get the best idle RPMs. Then I change the idle screws by exactly one quarter turn on either side until I have an acceptable idle RPM of 900-1000 RPM.

    Next I set the throttle adjuster screws. Without the engine running I visibly check that both throttle arms lift off the idle position at the same time or else I adjust the cable extension screws so I have the right slack in the cable and both throttle arms move off of idle at the same time. Done!

    When checking with my twin precision manometer it always looks pretty good.

    One thing about setting your cabs by looking at the 'vacuum'. You only get reaonable vacuum at idle. There is not much vacuum difference at higher RPMs with no load on the engine (means testing in neutral). To gain a bit more inside on vaccum and RPMs I mount my manometers on my tank bag and ride downhill. That's were I get useful vacuum readings. Which taught me that my static method of synching the carbs is pretty good.

    /Guenther

  14. #14
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    why why

    would you remove the carbs to set the idle static method??? seems like too much work to me ..Its really all about the cables and routing .. using the device set the idle spot on after idle jets are right then adjust cable on one side to as tight as possiable without raising idle .Then match the other all this done with carbmate hooked up ..... However its all moot mine changes when i ride at higher elevations ... Its a constant tweak like checking tire pressure Fuel injected airhead would be nice

  15. #15
    Polarbear Polarbear's Avatar
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    After 1000's;

    Had Airheads since 1972 and can say I've done so many, really cannot count 'em all. Love my Airheads. I have done several synch carbs with NO tool to read synch between the two, including my R100/7 since 1978. Yep, I have all the mercury sticks and TM tools. Just with practice, you may find yourself very good at this too. NOPE, I cannot possibly read or feel the balance in my head, just have got the feel down pat and have nailed it too many times with no tool. It takes practice, but many here have been on Airheads like me so long, may well know where I come from. USE the tools, its best. I just say these things(Airheads) become blood in our veins and are so in synch with their owners. Its cool, knowing a machine this well. I set to begin, with singling out one cylinder and idle screw out past the stop, no idle at all. Turning it in until you get your minimal idle set(per cylinder) and move on with cables. The cables need slack and should never be holding the idle, or you must start over. Just two tips, most probably know already. PS; Idle air mixture screw also needs to be optimized for its purpose. This whole carb setup gets very quick and easy with time in saddle. Sure nice having the carb knowledge on trips as the older bikes owners have a definite need to know these things, imo. A BING works great and for decades with just keeping them very tidy and cleaned up. Mine have 388000m(R100/7'78), same carbs. Randy

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